In color electrophotography, sequential transfers of individual color separation toner images build up a full color image. The nature of the electrophotographic technology allows sequential images to vary in content. Often it is desirable to put a protective clear layer on top of the image to make the image more durable. In the Kodak NexPress 2100 a 5th imaging module is used to apply a clear toner on top of a 4-color image in-line with the rest of the imaging process. In the Xerox iGen3 an inline coating apparatus is used to apply a clear overcoat on top of a 4-color image. In both of these devices the clear layer can also be applied in an image-wise fashion so as to create watermarks. In the Kodak device the clear imaging unit allows every sheet to vary in the placement of the clear layer, that is, the clear layer can be a different watermark for every sheet and can also vary between a watermark and a full clear coating. In the Xerox device the clear layer can not vary from image to image. Both of these devices require expensive additional hardware to enable the in-line capability to apply a clear layer, thus there remains a need to enable a low cost method of applying a clear layer on top of an image produced with an electrographic machine. An additional need is a low-cost method to apply a clear layer that can be varied in content from sheet to sheet.
The uses of clear, non-marking toner layers have been described in the prior art, for example in U.S. Pat. No. 5,737,677, filed in the names of Tombs et al., the contents of which are incorporated herein by reference, as aids to improved transfer, especially for high quality color electrophotography. They indicate that a clear toner underlay improves transfer efficiency over the whole gamut of toner layer thickness (optical density).
Still other apparatus for improving transfer with clear toner are described by Chowdry et al in U.S. Pat. Nos. 5,102,765 and 5,102,767. In Chowdry et al clear toner is transferred to a receiver and preferably fixed to the receiver. Thermal assisted transfer is then used to transfer a marking particle image onto the receiver which includes the clear fixed toner overlay. The role of the clear or uncolored toner layer is to serve as a thermoplastic layer so as to augment thermally assisted transfer of the marking particles.